Sheet metal piece parts such as used in automobiles, appliances, aircraft, farm implements, construction equipment, etc., are typically formed by a series of stamping operations in a multi-stage mechanical transfer press which manipulate the work piece to a desired shape and size. Each stamping operation makes use of a pair of dies which engage the sheet metal piece part and form it as desired.
In early multi-stage presses, each piece part was transferred manually from one stage to the next. This proved to be slow and dangerous and gave way to automated transfer arrangements where each piece part is sequentially moved into position in the various stamping stages. The piece part is engaged by a clamp or vacuum cups attached to mechanical linkage for moving the piece part between stamping stages. One common approach makes use of a tri-axis transfer drive mechanism employing a large and complicated arrangement of transfer lift/lower, feed/return, and clamp/unclamp cams. These cams, in turn, drive the combination of transfer feed, clamp and lift carriages which displace a pair of parallel, spaced feed bars in a timed manner. This approach suffers from the complex displacement sequence through which the lift bars are moved and the associated expensive and complicated drive system.
In an effort to improve on the tri-axis transfer drive mechanism, a cross bar transfer approach was developed. In this approach, one or more cross bars span the pair of parallel, spaced lift bars and have associated therewith a plurality of vacuum cup arrays. Each vacuum cup array is adapted to engage and transport a given piece part, allowing the cross bar transfer mechanism to transport a plurality of piece parts from one stamping station to the next.
Referring to FIG. there is shown an upper perspective view of a cross bar transfer press 20 of the prior art. The cross bar transfer press 20 includes a cam box 22 enclosing a transfer lift cam 24, a transfer feed cam 26, and various gears 28 for engaging and displacing first and second lift bars 30 and 32 and cross bars 42 and 44 via first and second control arms 34 and 36. No. 38 identifies a feed bar motion diagram illustrating the sequential displacement of the lift bars 30, 32 and cross bars attached thereto during each stamping cycle. A plurality of spaced cross bars extend between and are coupled to each of the first and second lift bars 30, 32, with first and second cross bars respectively identified as elements 42 and 44. Displacement of cross bars 42, 44 begins at the upper left of the feed bar motion diagram 38 and proceeds rightward to a position where the piece parts are deposited on a given die. Following deposit of the piece parts, the cross bars are displaced leftward to an intermediate position of the feed bar motion diagram 38. After the piece parts are stamped, the cross bars 42, 44 are displaced leftward by means of the control arms 34 and 36 to the starting point of the motion diagram.
Each of the first and second cross bars 42, 44 is provided with a plurality of vacuum cups 56 for engaging and supporting first and second piece parts 46 and 48. The piece parts are displaced in the direction of the arrow in the figure in a sequential manner to each of the stamping stations. Moving bolsters 52 and 54 support one or more lower dies 58 upon which each piece part is sequentially positioned. A plurality of upper dies (not shown in the figure for simplicity) positioned above each of the respective lower dies 58 is then displaced downward by a suitable press drive mechanism for stamping each piece part. The press is also used to drive the workpiece transfer mechanism shown in the figure by suitable gearing and power take-off units such as the transfer power take-off shaft 40. The press is positioned above the transfer mechanism shown in the figure and is supported and maintained in position by means of a plurality of spaced press uprights 50a, 50b and 50c. Before each piece part is formed by a series of stampings, a plurality of vacuum cups 60 engage each piece part and load it to the cross bar transfer press 20. Delivery is from a sheet washing unit 62. The piece parts are earlier separated by means of vacuum cups 64 for sheet destacking and are then delivered via a magnet belt 66 to the washing unit 62. The piece parts are initially arranged in stacks 70 on a pallet 72.
The cross bar transfer press 20 shown in FIG. 1 also suffers from various limitations. For example, the location of the lift bars 30, 32 as well as the cross bars 42, 44 attached thereto make it difficult to gain access to the lower dies 58 for repair or replacement. Furthermore, connection of all of the cross bars to and the driving of the cross bars by control arms 34 and 36 precludes individual adjustment of cross bar position at each stamping station. Also, each die pair must be precisely positioned relative to the other die pairs in the multi-stage cross bar transfer press 20 in order to ensure uniform stamping of all piece parts at every stamping stage. Finally, because piece part pass line height varies from press stage to press stage, it is desirable to adjust upper die lift travel depending upon piece part thickness. Minimizing die lift travel increases press speed. However, individual adjustment of displacement of the cross bar and vacuum cup transfer mechanism of transfer press 20 shown in FIG. is precluded because each transfer mechanism is mounted to the first and second lift bars 30, 32.